Field of the Invention
The present invention relates to a surface light source device for illuminating a back surface of a liquid crystal display element by at least two types of light sources having different characteristics, thereby displaying an image on the liquid crystal display element, and a liquid crystal display device including the surface light source device.
Description of the Background Art
A liquid crystal display element provided in a liquid crystal display device does not emit light by itself. For this reason, the liquid crystal display device includes a backlight device (a surface light source device) as a light source for illuminating the liquid crystal display element at a back surface side of the liquid crystal display element.
Moreover, the liquid crystal display element includes a color filter and transmits, through a color filter, only light having a part of wavelengths in light emitted from a fluorescent lamp for emitting white light in a continuous spectrum, thereby extracting display colors of red, green and blue to perform color representation. In the case in which only light in a part of wavelength bands is cut out of light source light in the continuous spectrum to obtain the display color, thus, a transmission wavelength band of the color filter provided in the liquid crystal display element should be set to be narrower if color purity of the display color is to be increased in order to enlarge a color reproduction range. For this reason, if the color purity of the display color is to be increased, there is a problem in that a quantity of light transmitted through the color filter is decreased, resulting in reduction in a luminance.
A fluorescent lamp to be generally employed as a light source of a backlight device has a light emission spectrum with a peak in a wavelength shifted to an orange color of approximately 615 nm in a red wavelength range based on a characteristic of a phosphor. If the color purity is to be increased in a wavelength region of 630 to 640 nm which is preferable as pure red in the red color, therefore, there is a problem in that the quantity of transmitted light is extremely reduced, resulting in significant reduction in a luminance. In order to enlarge the color reproduction range while minimizing a light loss caused by the color filter, it is necessary to employ a light source for emitting light having a narrow wavelength bandwidth. In other words, it is necessary to employ a light source for emitting light having high color purity.
As a measure for solving the problems, in recent years, there has been proposed a liquid crystal display device including, as a light source, a monochromatic LED or laser having a narrow wavelength width, that is, high color purity. In particular, the laser has excellent monochromaticity and high light emission efficiency. Therefore, it is possible to obtain a liquid crystal display device having a large color reproduction area, providing an image of high quality with a high luminance, and having low power consumption.
In the liquid crystal display device including the surface light source device, there has been desired technological development capable of reducing a manufacturing cost by implementing a surface light source device of high quality having a high color reproducing characteristic and less luminance unevenness and color irregularity by using light sources in a number which is as small as possible.
For example, WO2012/002029 discloses a liquid crystal display device including a directly under type backlight having a large number of LED light sources with lenses provided on a back surface of a liquid crystal display element. A lens considering light distribution is attached to each of LED light sources for R, G and B to be point light sources. Consequently, a uniform surface light source having luminance unevenness reduced is fabricated.
The surface light source device and the liquid crystal display device described in WO2012/002029 hold uniform luminance distribution by spreading and disposing the LED light source with the lens over a parallel surface with the back surface of the liquid crystal display element. For this reason, a large number of LED light sources are required so that a manufacturing cost is increased.
Furthermore, it is effective to increase the number of the LED light sources in order to enhance the luminance of the surface light source device. However, in order to cause the LED light sources to mix respective colors of light emitted therefrom so as to form a uniform white light, it is necessary to take the number and arrangement of the LED light sources into consideration, which is not easy.